The problems expected from activities in the Van Allen belts are mentioned by the solar-photovoltaic approach contractors, as well as NASA. For instance, Ref. All points out that degradation of the solar cells will occur in the radiation belts, as does Ref. A2. The orbit transfer times are estimated in several of the documents examined. Reference Fl (an early document) estimates a transfer time of 6 to 12 months, as does Ref. A2. Reference A4 (NASA) gives estimates of about 50 days with ion propulsion and 5 to 20 hours with chemical propulsion, while Ref. A9 (also NASA) estimates a one day transfer time with chemical propulsion and 75 to 90 days for electric propulsion approaches. The feasibility of chemical propulsion in this application depends on solving the added stress problems associated with the SPS package. Payload capacity of the transfer vehicle does not seem to have been considered in much detail and may, possibly, not be relevant. Reference F2 does state that a "space tug" can place in high orbit only 10% of the gross mass delivered to LEO, and Ref. A9 (NASA) gives payloads of 250 tons (225,000 kg) for a chemical transfer stage, 100 tons (90,000 kg) for 0 a nuclear/thermionic transfer stage, and 3,500 tons (3 x 10 kg) for an "SPS power dependent" vehicle. The large thruster diameter required for an ion propulsion system is cited as a key concern in several documents. Reference All states that current engine development at LeRC has concentrated on a 12 inch (30 cm) thruster, and that extension to 40 inches (1 meter) seems within technical feasibility. Reference A3, an early contractor document, mentions the need for 5 foot diameter ion thrusters. Reference All states that the grid material will be the limiting factor in the size, as the grids distort thermally during thruster operation. The choice of propellant for ion devices has also received some attention. Theoretically, any heavy element should be satisfactory. Argon is the preferred NASA propellant at this time but Ref. All prefers mercury. Use of mercury on the scale needed for the SPS may, however, result in contamination of SPS components. Reference E5, the NASA technology forecast for the period 1980 to 2000, predicts that nuclear electric propulsion may come into its own
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